Utility Meter offset defined by a time_period

[dgomes]

Description:

Breaking-change: offset is no longer an integer value dependent on the cycle, but a a very flexible time_period.

Currently offset configuration parameter is an integer that limits daily cycles to have offsets in days, monthly cycles to have offsets in months, etc.
This PR increases the flexibility of offset by making it a time_period that can be described in a combination of hours, days, months, etc. This makes it possible for a yearly cycle to start at a given day of a given month.

It causes a breaking change, but since the component is very recent (0.87) I believe users will acknowledge the improvement and fix the configuration.

Related issue (if applicable): fixes #20918

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#8459

Example entry for configuration.yaml (if applicable):

utility_meter: 
  energy_bill:
    source: sensor.energy
    cycle: yearly
    offset:
      days: 31  

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[MartinHjelmare]

Looks good. Just found a typo.

[MartinHjelmare]

Can be merged when conflict is solved and the breaking change is highlighted in a paragraph in the PR description.